A HSV-1 antigen preparation obtained by zwitterionic detergent-extraction of HSV-1 infected Vero cells was investigated for its ability to protect mice against establishment of latent infection in the trigeminal ganglion following HSV-1 challenge by ear scarification. The antigen preparation was shown to induce antibody production and correlation between pre-challenge antibody level and establishment of latency was found. Although immunization did not reduce the overall incidence of ear erythema following challenge the duration of erythema was significantly decreased. Latency in the trigeminal ganglion was also significantly reduced in vaccinated mice and in addition, a correlation was found between the duration of erythema and latency, there being significantly more latent infections in mice with erythema of three or more days duration, than in those with erythema lasting less than three days.